Apparatus for controlling the timing of periodically actuated switches



reamed June 19, 1951 2,557,740 APPARATUS FOR CONTROLLING THE TIMING OFATED SWITCHES Alexander Goldstein and Hans Blatter,

PERIODICALLY ACTU- tingen, Switzerland, assignors to AktiengcsellschaftBrown,

Boveri I; Cie, Baden,

Switzerland, a joint-stock company Application December 1, 1949, SerialNo.

In Switzerland Decemberll, 1948 Claims. (cl. car-4a) The presentinvention relates in general to electrical apparatus such as mechanicalconverters and the like which include periodically actuated switchingdevices for controlling the flow of current to a load, and is animprovement upon the converter apparatus described in our copendingapplication Serial No. 664,396, filed April 23, 1946.

In the art of mechanical rectifiers for example, it has beenconventional practice to incorporate an auxiliary control device such asa reactor in the primary side of the switch contacts for the purpose ofeffecting an interval of reduced primary current so that the contactsmay be opened under relatively arc-free conditions. As the contacts areopened and closed periodically by a contact bridge actuated by a powerdrive which is synchronized with the alternations in the alternatingcurrent supply, it is obvious that the timing of opening of the contactsis important and that contact opening should occur at the instant thereactors or other current reducing means have produced their optimumefiect.

In the above referred to application, control over the contact openingtime is obtained by deriving a measured control quantity the magnitudeof which varies as a function of the time interval between the beginningor alternatively the ending of the period of reduced current effected bythe reactor and the opening or alternatively the closing of therectifier contacts, comparing this control quantity with a referencequantity of fixed magnitude representative of the optimum condition toobtain the diiference if any, and utilizing such difierence to readjustthe timing of the contacts-until balance between the We comparedquantities is reattained. The apparatus for producing the measuredcontrol quantity comprises a discharge tube of the grid control typehaving its anode-cathode circuit closed and opened periodically in timedrelation with operation of the rectifier contacts, and its grid bias islikewise varied periodically in timed relation with operation of thereactor element. The arrangement is such that the grid bias, normally ata negative value sufiicient to maintain the tube in a nonconductivestate, is driven positive at the beginning of each interval of reducedcurrent and shortly before the rectifier load contacts controlling theanode-cathode circuit of the tube are set to open. Current will thusflow in the anodecathode circuit of the tube from the beginning of theperiod of reduced current until the rectifier contacts open. The averagevalue of these periodic pulses of anode-cathode current thus become ameasure of the time between the instant the reactor begins to producethe interval of reduced current and the instant at which the loadcontacts open.

While this system is generally satisfactory, it does have one operatingdisadvantage which stems from the fact that the anode-cathode circuitpasses through a switching device formed by one of the stationaryrectifier load contacts and the movable contact bridge, andtheoretically this anode-cathode circuit is broken at the instant thebridge lifts from the contact. In practice however it has been foundthat this arrangement for breaking the flow of current in the tube issatisfactory so long as the contacts operate in an arc-free manner.Arc-free operation cannot however always be ob tained especially inapplications where the load is subject to rapid fluctuations, and whenarcing does occur it is obvious that the anode-cathode circuit of thetube will not be interrupted at the instant of load contact break butrather at a somewhat later instant when the are between the contact andbridge has been extinguished. This then introduces a corresponding errorin measurement of the average value of the anode-cathode current for theperiod desired which is refiected by a like error in response of themechanism whose function is to adjust the timing of the contacts.

Moreover even under arc-free operating conditions of the load contacts,error in measurement is still possible because the contact bridge maynot always lift from both stationary contacts at the same instant withthe result that the rectifier load circuit through the contacts may bebroken while the anode-cathode circuit of the discharge tube may yet beunbroken.

The object of the present invention is to overcome the foregoingdisadvantage and the objective is obtained by providing an extra set ofswitching contacts operated synchronously with the load contacts of therectifier and whose sole function is to periodically open and close theanode-cathode circuit of the discharge tube component of the intervalmeasuring device.

One practical embodiment of the invention as applied to a polyphaserectifier system is illustrated in the accompanying drawing which showsthe improved system in schematic form.

Referring now to the drawing, the movable contact bridges of a polyphasemechanical rectifier are designated by al, a2 and a3 which coact withopposed sets of stationary contacts vi, 222, v3. The latter areconnected on one side to an alternating current network 17 throughcommutating reactors or chokes el, e2 and e3 and switch S, and on theother side to feed a direct current load system C.

The drive for actuating the several contact bridges is a synchronousmotor d connected to the network b. The motor shaft drives cams tl, t2and t3. which through cam follower rods ul, a2, a3 function toperiodically lift the contact bridges al, a2, 03 out of engagement withtheir associated sets of fixed contacts vi, v2, 123 in the propersequence as related to the phase sequence of the polyphase power supplyI).

The control quantity for regulating the contact timing is obtainedthrough the use of the discharge tube' h which is of the grid controltype. Its anode-cathode circuit is arranged to be opened and closed intimed relation with operation of the load contact bridges al, a2, a3,but instead of using the latter also as the switching device for theanode-cathode circuit as is done in the co-pending application SerialNo. 664,396, an auxiliary set of contacts and bridge are employedespecially for such purpose. The auxiliary contacts are designated :24and the associated auxiliary contact bridge al. Bridge a4 is actuatedperiodically in a manner similarly to the other contact bridgesassociated with the load contacts by means of cam t4 and cam followerrod a4 and is synchronized with the movement of contact bridge 03.

The grid of tube h is normally biased negatively from an auxiliaryvoltage source i and the grid circuit extends from the source 2 througha winding f inductively associated with the reactor e3. The use ofreactors in the primary of the rectifier is well known and hence thetheory of their operation need not be detailed here. It is suflicient tosay that at the beginning of the interval of reduced current effected byreactor e3, the inductive effect inherent therein is sufflcient toinduce a potential pulse in winding 13 which is so connected in the gridcircuit of tube h that the grid bias is swung positive and renders thetube conductive. As this action takes plate at a time shortly before thecontact bridges a3 and a4 operating synchronously are set for liftingfrom the respective sets of fixed contacts v3 and v4, current will flowin the anodecathode circuit of tube h from the beginning of the intervalof reduced current in the phase served by load contacts 03 until thelatter and also contacts 04 open. Thus as the rectifier operates and theload and auxiliary contacts periodically open and close, periodic pulsesof direct current of constant amplitude flow in the anode-cathodecircuit of tube It and hence it now becomes obvious that the averagefiow of current in this tube circuit, which can be observed by anammeter g placed in the circuit, is a measure of the time between theinstant at which the choke e3 begins to produce its interval of reducedcurrent and the instant at which load contact bridge a3 lifts from itsfixed contacts 03.

To measure the average current in the tube circuit a resistance It isconnected therein and the drop in voltage across the resistance willhence vary with the current.

The potential across resistance It is used to regulate the timing of therectifier contacts so that they always open at the optimum condition, i.e. substantially maximum current reduction in the primary line. For thispurpose the voltage drop at resistance It is fed into an amplifyingdevice I which is of conventional construction and therefore has beenillustrated in block schematic only. Connected across the output ofamplifier l is a resistor m that is connected to a selected portion of amanually adjustable voltage dividing resistor n in such manner as toderive a voltage which the respective voltage sistors.

An instrument type relay 9 of the crossed coil type is used in thecontrol and the contacts of this relay control the operation of areversible motor q. Coil pl is connected directly to an auxiliary sourceof direct current power indicated by conventional symbols and hencecurrent through it remains constant. However coil :22 is connected inthe circuit between opposed resistors 1n and n and hence its currentdepends upon the unbalance between the voltage drop across these tworesistors. The arrangement is such that when the current through coil plof the instrument's moving system is equal to the current through coilp2 which is at right angles to coil pl, the contact arm p3 will take aposition between and spaced from the stationary contacts 114, p5 and themotor q will stand still since the circuits to both of the motor fieldsqi and 112 are open. If however the two coil currents are unbalanced,the contact arm 93 will be moved into engagement with one or the otherof contacts pl, p5, dependent upon the direction of the unbalance, andhence will cause motor q to run in one direction or the other asdetermined by the alternatively energizable motor fields qi and :12.

A control mechanism 1' actuated by the shaft of motor q functions toadjust the timing of the contact bridges al-al relative to theirrespectively associated phase voltages of the polyphase power supply bychanging the positions of the cam follower contact bridge actuating rodsu l-ul relative to their actuating cams to tl-tl in one direction inwhich motor q is rotated.

The various operating components are adjusted so that when the loadcontacts of the rectifier are operating when optimumn condition, i. e.without arcing, the drop across resistance 1: and hence the resultingdrop across resistor m is such that the current through relay coil p2 isequal to that through coil pi and hence motor q will stand still.However should the desired timing of the rectifier contact bridge a3relative to the operation of the commutating reactor e3 shift in onedirection or the other which would therefore cause the contact bridge a3to are as it lifts from the contacts '03, such shift will appear as achange in the conducting time of tube It and hence change the voltagedrop across resistor k. In turn the current through instrument relaycoil :12 is changed and this causes relay p to effect rotation of motorq in such direction as will readjust the timing of the contacts to theoptimum operating condition. When the correct timing is reached, thevoltage drops across resistor R: and m will have gained a rebalance andmotor q will stop. The regulating action is thus entirely automatic andconstantly maintains correct timing of the contact bridges.

Moreover since a separate set of switching contacts operatingsynchronously with one of the sets of load contacts on the rectifier areutilized solely to open and close the anode-cathode circuit of thecontrol tube It, correct measurement of the interval between thebeginning of the period of reduced load current and the actual dropsacross the two rerepresents the difference in Another possiblemodification would be to associate the winding 1 with reactor e2 or elinstead of e3 as now illustrated leaving the auxiliary set of contactsa4 v4 synchronized with contacts a3 03 of e3. In such case however amean value proportional to a constant value if el) plus the time betweenmeasuring instrument 9.

We claim:

1. A mechanical current converter comprising a set any deviation in theaverage anode-cathode circuit of said periodically in a predeterminedsequence to peri odically open and close the several phases connected tosupply a common load, a control device in series with each set of loadcontacts for effect ing periodic intervals of reduced current to minitorestore the average current to said selected value.

control device controlling the grid bias of said tube.

5. A mechanical converter as defined in claim REFERENCES CITED Thefollowing references are of record in the file of this patent:

UNITED STATES PA ENTS Number Name Date 1,529,341 Burnham Mar. 10, 19252,279,729 Bedford Apr. 14, 1942 2,465,682 Goldstein Mar. 29, 1949

